Three main topics will be investigated during the coming year: 1. Antigenic modulation of G-MuLV positive leukemic cells - These cells lose their membrane antigenic expression when transplanted into W/Fu rats that have been made tolerant by neonatal injection of deaggregated antigens. The mechanisms of antigenic deletion will be studied with the use of immunoelectron microscopy and a battery of immunologic tests. 2. Induction of unresponsiveness to leukemic cells - This state of specific tolerance can be induced by three different methods originated in this laboratory. They will be comparatively studied and the participation of T and B lymphocytes in these processes will be analyzed. 3. Patterns of metastases in viral and chemically-induced leukemia - The mechanisms of tumor spread and selectivity of metastatic sites will be comparatively analyzed with the use of Cr-51 labeled leukemic cells of the two different cell lines.